"Unless you are an elite athlete, it doesn't matter," said Miriam Nelson, director of the John Hancock Center for Physical Activity and Nutrition at Tufts University, in an e-mail. "The most important thing is to find a time of day that you like best and when you can exercise most consistently."
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And this advice is especially true for aerobic exercise like brisk walking or cycling. For aerobic workouts, "it doesn't much matter when you do it. Basically, it's whenever you can," said Michael R. Deschenes, a physiologist and neurobiologist at the College of William and Mary in Virginia.
For building muscle strength through weight training, though, "you're better off in the late afternoon or early evening because core body temperature is higher and muscles perform better when core temperature is higher," Deschenes said.